1. Field of the Invention
This invention relates to a light visible phase-locked array, which can be utilized in an optical measurement, an information processing and the like, and a method of manufacturing the same.
2. Description of the Prior At
Recently, there is an increasing demand for a semiconductor laser in various fields of industry. To meet these demands, the semiconductor lasers made mainly from GaAs based semiconductor or InP based semiconductor have been actively commercialized. In particular, the demand for a semiconductor laser of high output is so persistent that, in the case of a GaAs based semiconductor laser, a product having a light output exceeding 50 mW has been developed.
However, a material for the semiconductor laser has a limitation with respect to the degree of the light density which can be allowed to emit from the out-going side of the material. Since the laser may be destroyed as the light density exceeds this limitation, the maximum light output of the semiconductor element is determined depending upon the size of the sectional area of light beam at the out-going end face of the material. Thus, in the case of a stable fundamental transverse-mode laser, since its transverse-mode thereof is stabilized by confining light in a light waveguide channel of a small sectional area, the laser is structurally unsuited for enhancing the output thereof, and its output is limited to the order of 100 mW.
Therefore, in order to obtain a stable transverse mode as well as high out-put, a structure of so-called a phase-locked array has been developed. With this semiconductor laser, a stable transverse mode is realized by employing a plurality of adjacent waveguide channels and combining light phases in each of the waveguide channels. Moreover, since the output of the phase-locked array can be given by the sum of the outputs derived from each of the waveguide channels, it is possible to increase the light output by increasing the number of the waveguide channels 1
FIGS. 5a to 5c illustrate the classification of the phase-locked arrays, which are classified according t6 the guiding mechanism. As in the case of the single guiding channel employed in the ordinary semiconductor laser, a gain guided type (FIG. 5a), a loss guided type (FIG. 5b) and an index guided type (FIG. 5c) are mainly employed. In these Figures, reference numeral 10 denotes GaAs substrate, 11 a first clad layer, 12 an active layer, 13 a second clad layer, 15 p-type Al.sub.0.18 Ga.sub.0.82 As layer, 17 a cap layer, 18 a blocking layer, 19 an element region and 20 an inter-element region.
In the gain guided type laser (FIG. 5a), the refractive index in a region having a gain is smaller than that in a region having no gain (non-gain region) as shown in FIG. 6a, whereas in both of the loss guided type laser (FIG. 5b) and the index guided type laser (FIG. 5c), the refractive index in a region having a gain is larger than that in a region having no gain (non-gain region) as shown in FIGS. 6b and 6c.
Additionally, there is also an anti-guided type hereinafter explained with reference to FIGS. 9 and 10. This anti-guided type is featured as shown in FIG. 6d in that the refractive index in a region having a gain is smaller than that in a region having no gain, which is quite different from the refractive index distributions the loss guided type laser (FIG. 5b) and of the index guided type laser (FIG. 5c).
In GaAs-based semiconductors, the gain guided type (FIG. 5a) and the loss guided type (FIG. 5b) have been widely utilized. Likewise, in the phase-locked arrays, lasers of these types are widely utilized, because the same manufacturing process as that of the GaAs-based semiconductors can also be utilized in the phase-locked array. A laser of the index guided type is also widely utilized as it can be manufactured by forming a simple ridge stripe structure.
As for the coupling of waveguide channels in the phase-locked arrays, there are two coupling modes; namely, 0.degree. phase coupled mode wherein the phase of a waveguide channel coincides with the phase of the neighboring waveguide channel, and 180.degree. phase coupled mode wherein the phase of a waveguide channel is shifted from the neighboring waveguide channel by 180.degree.. In the case of the 0.degree. phase coupled mode, it is possible to obtain a single lobe far field pattern as shown in FIG. 7a, so that it is easy to obtain a parallel light beam or to condense the light beam. Whereas in the case of the 180.degree. phase coupled mode, a twin lobe far field pattern will be resulted as shown in FIG. 7b, so that it is rather difficult to utilize the mode.
In the structure of the loss guided type laser (FIG. 5b) or the index guided type laser (FIG. 5c), the refractive index n.sub.1 in the element region is higher than the refractive index n.sub.0 in the inter-element region as described above. Accordingly, when the structure of the loss guided type or the index guided type laser is employed, light beam thereof is fundamentally forced to be confined within the element region, so that the coupled mode thereof may most likely be of the 180.degree. phase coupled mode as shown in FIG. 8a.
The reason of the tendency of the structure of the loss guided type or the index guided type to take 180.degree. phase coupled mode can be attributed to the fact that since the intensity of light in the inter-element region is low]the light intensity at the center of the inter-element region is approximately zero, so that the 180.degree. phase coupled mode is much favored in view of less degree of the waveguide loss and a higher stability of light.
In the structure of the gain guided type laser (FIG. 5a) , the refractive index distribution thereof indicates that the refractive index in the element region is slightly less than that in the inter-element region as shown in FIG. 6a. Therefore, light is weakly confined in the element region, thus the 0.degree. phase coupled mode is more favored to occur. However, since this structure of the gain guided type laser is designed to obtain the light solely through a gain, this phase coupled mode is rather unstable against temperature and optical output.
On the other hand, in the anti-guided type laser, the refractive index distribution thereof indicates as shown in FIG. 6d that the refractive index in the element region is lower than that in the inter-element region as the case of the gain guided type. However, this anti-guided type laser is more advantageous than the gain guided type laser in that a very strong coupling of light can be obtained between the neighboring element regions due to a large difference in defractive index between the element region and inter-element region, so that the 0.degree. phase coupled mode can be more easily obtained as shown FIG. 8b.
To be more specific, there is a gain in the element region so that light is confined in the element region, while in the inter-element region, the refractive index therein is higher than that in the element region that light is confined also in the inter-element region. As a result, the coupling of light occurs in the element region as well as in the inter-element region, thereby allowing the 0.degree. phase coupled mode to be easily obtained.
In the followings, the anti-guided type will be discussed more in detail. FIG. 9 schematically shows a perspective view of the elemental structure of the conventional anti-guided type phase-locked array, and FIG. 10 shows an enlarged sectional view of the main part of the phase-locked arrays shown in FIG. 9 (Appl. Phys. Lett. 55 (1) , July 1989) .
As shown in FIGS. 9 and 10, on a GaAs substrate are formed in succession a first clad layer 11 of n-type Al.sub.0.4 Ga.sub.0.6 As, an active layer 12 constituting a GaAs/AlGaAs multiple quantum well layer, and a second clad layer 13 of p-type Al.sub.0.4 Ga.sub.0.6 As. The second clad layer 13 is etched into a mesa structure thereby forming an element region 13a through which electric current is to be passed into the active layer 12, and an inter-element region 13b through which electric current is not to be passed into the active layer 12. In this case, the refractive index n.sub.1 in the inter-element region is higher than the refractive index n.sub.0 in the element region.
On the mesas-shaped second clad layer 13 is selectively formed a p-GaAs layer 14. Further on all surface of the second clad layer 13 including the p-GaAs layer 14 is formed a p-Al.sub.0.18 Ga.sub.0.82 As layer 15, on which a p-Al.sub.0.4 Ga.sub.0.6 As layer 16 is further formed. Reference numeral 17 represents a cap layer formed on the p-Al.sub.0.4 Ga.sub.0.6 As layer 16.
In this device, AlGaAs layer 16 is formed over the element region 13a and the inter-element region 13b. In particular, in the inter-element region, this AlGaAs layer 16 is formed close to the active layer 12 . Whereas in the element region, this AlGaAs layer 16 is formed remote from the active layer 12 . Therefore, the refractive index in the inter-element region is higher than that in the element region as shown in FIG. 10. Moreover, since there is a large difference in defractive index between the element region and inter-element region, it is possible to attain a highly stabilized mode coupling.
Electric current is selectively caused to flow into the element region. In this occasion, oscillations are caused to occur not in the inter-element region due t6 the large radiation loss therein of the optical mode, but only in the element region. In order to maximize the coupling between the neighboring element regions in this construction, the width "s" of the inter-element region is set to approximately a half of the resonance wavelength lateral wavelength of light as follows (IEEE J. Quantum Electron 26,482, 1990): EQU s=.lambda..sub.1 /2 EQU .lambda..sub.1 =.lambda./(n.sub.1.sup.2 -n.sub.0.sup.2 +(.lambda./2d).sup.2).sup.1/2
wherein s is a guide region spacing; d is stripe width; .lambda. is wavelength; .lambda..sub.1 is lateral wavelength; n.sub.0 is an effective refractive index in the element region; and n.sub.1 is an effective refractive index in the inter-element region.
When the width "s" of the inter-element region is set in this manner, the array mode turns into a resonance state, so that all of the elements couple to each other the 0.degree. phase coupled mode, and take a state of the highest array mode gain (i. e. a state wherein the oscillation of the 0.degree. phase coupled mode is most likely to occur).
However, even with this conventional anti-guided type phase-locked arrays, it is difficult to achieve a stable 0.degree. phase coupled mode. The reasons for this can be explained as follows.
FIG. 11 illustrates the gain of each of the modes in relative to the width "s" of the inter-element region the number of the element is set to 10.
As seen from FIG. 11, when s=.lambda..sub.1 /2, the gain for the 0.degree. phase coupled mode is in a high level as indicated by the solid line. However, in addition to the 0.degree. phase coupled mode, there are also a 180.degree. phase coupled mode and an adjacent mode as indicated by broken lines. In this case, since it is difficult to enlarge the gain difference g.sub.p between the gain of the 0.degree. phase coupled mode and the gain of either the 180.degree. phase coupled mode or the adjacent mode, the shifting of mode, i. e. from the 0.degree. phase coupled mode to the 180.degree. phase coupled mode or to the adjacent mode may be frequently caused to occur.
In other words, the elements of the phase-locked array may be producing a laser of various modes in a disordered manner, or the mode (zero-order mode or first-order mode) to be selected by each element may be flactuated depending on a light output, or becomes highly unstable as shown in FIGS. 12a to 12d.
Therefore, in order to obtain only the 0.degree. phase coupled mode, it has been required to take some measures, such for example as to mount a Talbot filter (Appl. Phys. Lett. 5(1), Jul. 3, 1989) in separate to the phase locked laser arrays.